A system for monitoring the pressure of a tire of a vehicle wheel serves to indicate incorrect pressure to the on-board computer of the vehicle in order for it to alert the driver promptly.
A system of this type, attached to the valve inside the tire is thus inaccessible and power must be drawn in moderation from the battery which supplies it. To this end, the pressure-value radio transmissions are discontinuous, with, for example, a cycle of six minutes when the vehicle is moving and one hour when stationary. It is thus necessary to ensure that the result of each measurement is well received in order for the cycle of information not to be extended undesirably.
To this end, each measurement is transmitted with redundancy in the form of a series of a number of bursts of bits of information relating to the pressure. For example, in a standard manner, three bursts of 50 ms are transmitted with a spacing of about looms. Each wheel thus transmits its data in a random manner with respect to the others, and possible collisions in the transmissions of two wheels do not generally affect the totality of the bursts of a series. The redundancy, ie. the number of repetitions of transmission of each measurement, is thus determined according to the random risk of such radio collisions.
The Applicant has noted, however, that above a certain vehicle speed, the correct reception of the bursts was no longer assured, this means that a number of bits of each burst were erroneous and this caused the total loss of each burst. The redundancy lost all effectiveness. It is even possible to speak in terms of fading of the transmission data.
The Applicant has thus observed, especially at high speeds, that upon each rotation of the wheel, the pressure-monitoring system temporarily lost radio contact with the on-board computer over a specific angular sector linked to the presence of metal masses of the chassis and of the wheel, forming a screen. This was therefore not a question of random events, which explained the ineffectiveness of the redundancy.
Thus the Applicant proposes the following solution to the problem of transmission of remote readings from a tire of a wheel.